Like DC I am new to this & still planing but he offers some good advice from what I have learnt so far. What materials & components you use will depend on what you intend to cut with the machine. What sort of material will you be cutting as the more experienced members will ask you this.
20mm has been used for a few builds but you need 20mm plate not 20mm flat bar. What else will you be using for the gantry construction other than the aluminium plate?
What experience of machining or what tools you have available will make a difference.